Rear Disc Brakes – All Bound Up

Better Binders out Back in a Day

When it comes to discussing the history of Mopar, the first thing that everyone remembers and talks about is the fact that Mopar knew how to go quick and win races. The cars were often in the winner’s circle, and were unsurpassed in their ability to accelerate. But, what no one seems to remember is the sometimes less-than-adequate braking capabilities of these cars. Sure, in stock form, a ’70 Road Runner was aptly capable of stopping while driving through town–in 1970, but in today’s traffic…

There are a lot of reasons you should consider when it comes to upgrading to disc brakes. The main reason is safety. For starters, drum brakes are easily damaged by heat and moisture. Get them wet or hot, and they will not work–period. Even when they’re in good working order, they offer less-than-optimal stopping power, especially in today’s traffic conditions. Chrysler actually introduced standard disc brakes on the front of their vehicles sometime in the 1950s, but it wasn’t until the mid-1960s, when power-assisted units, which made disc brakes easier to operate, hit the market. Some modern vehicles still come with drum brakes on rear wheels. Rear drum brakes are cheaper to produce than disc brakes, and since a vehicle’s front brakes actually do 70 percent of the work, rear brake performance can be sacrificed for cost.

Since drum brakes do have their limiting factors, and adding front disc brakes to your car is definitely an upgrade in safety and performance, is an upgrade to rear disc brakes worth doing on your car? This is a question that only you can answer for yourself. Some guys feel that rear drum brakes are more than sufficient for stopping their cars, and it’s true that a good working drum brake system on the rear of a car can work properly during normal driving conditions–if you have front disc brakes. If you have drum brakes on all four corners, just think back to the last time you panic-braked when some idiot pulled out in front of you, and tell me that an upgrade isn’t worth it.

It goes without saying that you can’t install rear disc brakes on your car if the front has drums on it. Well, you could but when you hit the brakes, your rear tires will lock up before the front ones even start to slow down. But if a car equipped with front disc brakes and rear drum brakes is sufficient, why upgrade to rear disc brakes? For starters, the front discs can only do so much. Have you ever noticed that your rear drum shoes last much longer than the front shoes/pads? We’ve been told that by adding rear disc brakes to your car, you can effectively add as much as 30 percent more braking force to your braking system. This means you stop a lot quicker, which could come in handy when that idiot pulls out in front of you.

But, you might be worried that adding a rear disc-brake kit to your car is going to be too expensive, and granted, you will have to shell out a few hundred dollars for a kit that works on your car. But, how much would the bodywork be if you actually ran into the aforementioned idiot?

We’ve had a couple of close calls in our Dart Sport, and it scared us enough to convince us to do some research about what’s available in regards to a rear disc-brake kit. But like you, we were more than a little concerned with what these kits cost. Let’s face it, there are a lot of kits out there, and depending on the options you choose, it can get pricey really fast. We actually stumbled across Master Power Brakes’ website during our research, and were pleasantly surprised to find a basic rear kit for our Sport that wasn’t as expensive as a lot of other kits. The kit from Master Power Brakes included 11-1/8-inch multi-pattern vented rotors (this means you can use them with either 4-inch or 4-1/2-inch bolt pattern wheels), single-piston calipers with pads and integral parking brake, the required brackets, hoses, and cables. It was a complete kit for a fair and economical price. But, if we installed them, would we see a significant improvement in braking distance. The only way to find out was to order the kit and put it to the test. Before we started the install, we took our car to the local industrial-park-straight-stretch-of-road-testing facility after 5:00 pm, and set up a test. We decided that we would get the Sport up to a speed of 60 mph, and when we got to a marked spot on the road where we would start our measuring, we would hit the brakes and see what happened. With our rear drum brakes, the Sport managed a stopping distance of 133 feet and 6 inches. Apparently that was fairly close to where everyone thought it should be, but what would happen when we installed the rear disc brake kit? For those results, you’ll have to check out the end of our article.

According to the guys at MP Brakes, if your vehicle was originally equipped with disc/drum brakes, it would have come from the factory with a combination valve that includes an internal metering valve. Metering valve delays the pressure sent to the front calipers, because the springs in the rear drums do not allow the shoes to engage as quickly. This “delay” allows the shoes in the rear drums to engage slightly before the front calipers, thus reducing nose dive. For a disc/disc system, you do not want to use a metering valve. If your car has a combination valve with an internal metering valve, the combination valve should either be replaced with one that does not include a metering valve, or the metering valve could be removed and the opening plugged with a correct size plug.

Editor’s note: To remove the metering valve, first remove the cap and plug at the end of the combination valve. After that, simply remove the metering valve, and then reinstall the plug and Loctite it in place.

Location, Location, Location

There are different theories, rumors, or just outright guesses when it comes to the actual position of the caliper in relationship to the rotor. We’ve heard guys say that the brakes work better with the caliper behind the center line of the rotor. Some say it’s better if in front of the center line. And yet others say it doesn’t matter. We have even seen situations where the calipers are positioned at the very bottom location of the brake discs. The thinking there is that by binding the lowest point possible on the rotor, you are also creating the lowest possible center of gravity for the wheels. The calipers used on the MP kit are sourced from GM, and the design of these calipers is to mount them aft of the axles. This allows the parking brake cables to feed in from the front. According to our contact at MP Brakes, “We suspect the choice of location [from the OE], was chosen more for packaging than anything. Locating both front and rear calipers to the inside (behind the spindles on the front and in front of the rear axles on the rear) would provide a slightly lower center of gravity, and there might also be a slight advantage in the polar moment of inertia, but any difference would be so slight that we doubt any difference could be noticed, especially on a street car.”

Terms You Need To Know

Single-Piston Caliper Single-piston calipers have been the standard in braking for years. Single-piston calipers are very simple, and perform well in most applications. With the single-piston design, the disc brake rotor is squeezed between a pad that is stationary on one side, and a pad that is moved by the caliper’s piston on its other side. The advantage of this design is that it’s very simple, and is not prone to leaking or failure.

Four-Piston Caliper The four-piston caliper was the original design used by Chrysler. This system had one major weakness–four-caliper pistons had four times the chance of leaking or failing since there were four of them. Long term function without failure was the driving force to go to the single piston caliper design. An advantage of newer designed four-piston calipers is that the caliper is less prone to flexing under stress as opposed to early designs, which keeps the pads more evenly pressed on the rotor, producing more friction and stopping power. With today’s four-piston calipers, the leaking problem can be virtually eliminated or severely reduced by having the caliper’s piston bores sleeved with stainless steel. This eliminates bore corrosion, which leads to leaking.

Rotor Size Efficient braking requires the conversion of your car’s kinetic energy into heat. This heat is produced by the rotor being clamped between the disc brake pads. Fast dissipation of this heat is critical to achieving good brake function. In essence, the larger the rotor, the more efficiently it will dissipate heat, which translates into better braking. For most street applications, an 11-inch rotor is more than sufficient.

Vented Rotors Vented rotors are essentially two parallel discs with airspace between them. Between the two discs are air-moving fins that hold the two discs together. The function of these fins is to increase the speed in the rotor cools.

Cross-Drilled Rotors Better braking performance is all about cooling the rotor as fast as possible. When a rotor is cross-drilled, these holes help to discharge the heat and gases that build up within the rotor, and therefore cools it faster and clamps better. We need to make note that some guys feel that cross-drilling a rotor does increase its tendency to develop heat cracks that develop from the drilled holes.

Slotted Rotors When friction is produced by the clamping of the caliper, a gas is also emitted along with heat from the pads. The theory is, these slots help remove the gases that develop under the pads. Although cross-drilled and slotted rotors are not an absolute necessity for street use they are a relatively inexpensive upgrade when adding disc brakes.